/*
 * Copyright (C) 2007 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.collect;

import com.google.common.annotations.GwtCompatible;

import java.io.Serializable;
import javax.annotation.CheckForNull;

import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * An ordering that treats {@code null} as less than all other values.
 */
@GwtCompatible(serializable = true)
@ElementTypesAreNonnullByDefault
final class NullsFirstOrdering<T extends @Nullable Object> extends Ordering<@Nullable T>
        implements Serializable
{
    final Ordering<? super T> ordering;

    NullsFirstOrdering(Ordering<? super T> ordering)
    {
        this.ordering = ordering;
    }

    @Override
    public int compare(@CheckForNull T left, @CheckForNull T right)
    {
        if (left == right)
        {
            return 0;
        }
        if (left == null)
        {
            return RIGHT_IS_GREATER;
        }
        if (right == null)
        {
            return LEFT_IS_GREATER;
        }
        return ordering.compare(left, right);
    }

    @Override
    @SuppressWarnings("nullness") // should be safe, but not sure if we can avoid the warning
    public <S extends @Nullable T> Ordering<S> reverse()
    {
        // ordering.reverse() might be optimized, so let it do its thing
        return ordering.reverse().nullsLast();
    }

    @SuppressWarnings("unchecked") // still need the right way to explain this
    @Override
    public <S extends T> Ordering<@Nullable S> nullsFirst()
    {
        return (Ordering<@Nullable S>) this;
    }

    @Override
    public <S extends T> Ordering<@Nullable S> nullsLast()
    {
        return ordering.<S>nullsLast();
    }

    @Override
    public boolean equals(@CheckForNull Object object)
    {
        if (object == this)
        {
            return true;
        }
        if (object instanceof NullsFirstOrdering)
        {
            NullsFirstOrdering<?> that = (NullsFirstOrdering<?>) object;
            return this.ordering.equals(that.ordering);
        }
        return false;
    }

    @Override
    public int hashCode()
    {
        return ordering.hashCode() ^ 957692532; // meaningless
    }

    @Override
    public String toString()
    {
        return ordering + ".nullsFirst()";
    }

    private static final long serialVersionUID = 0;
}
